1. Field of the Invention
This invention relates to an image forming apparatus of the electrophotographic type or the electrostatic recording type, and particularly to an image forming apparatus in which a voltage applied to a transferring member disposed in opposed relationship with an image bearing member is controlled.
2. Related Background Art
FIG. 8 of the accompanying drawings schematically shows the construction of a popular image forming apparatus. The image forming apparatus shown in FIG. 8 is a printer of the transfer type utilizing the electrophotographic process.
A drum-shaped electrophotographic photosensitive member (photosensitive drum) 1 as an image bearing member is rotatively driven in the direction of arrow R9 at a predetermined peripheral speed (process speed), and image forming process such as electrifying image exposing, developing, transferring and cleaning are applied to this photosensitive drum 1.
That is, the photosensitive drum 1 is rotatively driven and the surface thereof is uniformly electrified to a predetermined polarity and predetermined potential by an electrifying roller (primary electrifier) 2. In the present example, a description will be made of a case where a photosensitive drum minus-electrified is used.
Next, image exposing by image exposing means (such as a scanning exposing apparatus of an image-modulated laser beam) 3 as image information writing means is effected on the electrified surface of the photosensitive drum, whereby the electrifying potential of an exposed light portion is attenuated and an electrostatic latent image corresponding to the exposed image information is formed on the surface of the photosensitive drum 1.
This electrostatic latent image is sequentially developed (visualized) as a transferable image) at a developing region N6 by the developing sleeve 4a of a developing device 4.
The thus developed toner image is transferred to a recording material P at a transferring region (transfer nip part) N5 by transferring means. The transferring means in the present example is transferring means of the contact transfer type using a transferring roller (contact transferring electrifier) 9.
The transferring roller 9 is comprised, for example, of a mandrel and an elastic layer of medium resistance formed around the mandrel, and is brought into pressure contact with the photosensitive drum 1 with a predetermined pressure force against the elasticity of the elastic layer to thereby form the transferring region N5. The transferring roller 9 is rotated in a forward direction relative to the rotation of the photosensitive drum 1 in the direction of arrow R9, i.e., the direction of arrow R10, substantially at the same peripheral speed as the peripheral speed of the photosensitive drum 1.
The recording material P is fed from a sheet feeding cassette (feeding means portion) 13 and is subjected to timing adjustment and fed to the transferring region N5 by registration rollers (not shown) disposed at the upstream side of the transferring region N5. That is, the registration rollers feed the recording material P to the transferring region N5 at such timing that just when the leading edge portion of the area of the toner image formed on the surface of the photosensitive drum 1 has arrived at the transferring region N5, the leading edge portion of the recoding material P also arrives at the transferring region N5.
The recording material P fed to the transferring region N5 is nipped and conveyed through the transferring region N5 with its surface kept in close contact with the photosensitive drum 1. Also, during the time from after the leading edge portion of the recording material P has arrived at the transferring region N5 until the trailing edge portion thereof leaves the transferring region N5, a predetermined transferring bias voltage of the plus polarity is applied from a transferring bias application voltage source (not shown) to the mandrel of the transferring roller 9.
In the process of the recording material P being nipped and conveyed through the transferring region N5, the toner image on the photosensitive drum 1 is sequentially transferred onto the recording material P by the action of a transferring electric field formed by the transferring roller 9 and the pressure force at the transferring region N5.
When it leaves the transferring region N5, the recording material P is separated from the surface of the photosensitive drum 1 and is conveyed to a fixing device 12, whereby the transferred toner image thereon is fixed as a permanent image on the surface of the recording material P, whereafter the recording material P is discharged as an image-formed article (a copy or a print).
The photosensitive drum 1 after the separation of the recording material P therefrom has adhering contaminants such as any residual lower and paper dust remaining on its surface removed by the cleaning blade 10 of a cleaner 11, and is used for the next image formation.
As the image forming process, there are, for example, a regular developing process in which the electrified surface of the photosensitive member is exposed correspondingly to the background portion of image information (a background exposing process), and the other non-exposed portion than the background portion is developed, and a reversal developing process in which conversely the electrified surface of the photosensitive member is exposed correspondingly to an image information portion (an image exposing process) and the exposed portion thereof is developed, and these are used with their features utilized efficiently.
During the time from after the recording material P passes through the transferring region N5 until the next recording material P arrives at the transferring region N5, in order to prevent the unevenness of the potential of the photosensitive drum and to quicken the rising of the transferring voltage in the case of the next recording material, a voltage is applied from the transferring bias application voltage source to the transferring roller 9. As this voltage, a voltage (non-sheet passing portion bias) smaller than that during transfer is applied. This is for preventing the flow of an excessive current to the photosensitive drum 1 by a transferring bias voltage equal to that during transfer being applied in the absence of the recording material P and further, preventing the photosensitive drum 1 from being plus-electrified.
Heretofore, this non-sheet passing portion bias has been fixed to one which minimizes the influence upon the photosensitive drum 1, such as the discharge starting voltage of the photosensitive drum 1 or less. This also holds true of an image forming apparatus using, for example, a recording material conveying belt, a recording material conveying drum or the like besides the transferring roller 9.
Now, the above-described transferring roller 9, the recording material conveying belt, the recording material conveying drum, etc. change greatly in resistance. Therefore, in an apparatus wherein the non-sheet passing portion bias is made constant, the amount of plus electrification received by the photosensitive drum 1 may become excessive when the resistance of these members lowers, and even after the photosensitive drum is primary-electrified by the electrifying roller 2, a predetermined amount of electrification cannot be maintained and the electrifying potential may sometimes become small.
In such case, in reversal developing, there is formed an image high in density over a round of the drum from the leading edge of the recording material P, and in regular developing, there is formed an image low in density over a round of the drum from the leading edge of the recording material P.
Further, when DC electrification is used, charges electrified to electrifying potential or higher in DC electrification cannot be eliminated and therefore, when the resistance of the recording material P rises due to an environmental fluctuation or an endurance fluctuation, white spots are created in the reversal developing by the bad charge elimination of an excessively electrified portion, and black spots are created in the regular developing and further, uneven electrification occurs to a halftone image or the like.
In contrast, heretofore, this has been coped with by effecting ante-electrification exposure or an ante-electrification preliminary charging process.
According to this, however, an apparatus (instrument) for effecting the ante-electrification exposure or the ante-electrification preliminary charging process is necessary, and this has led to the problem that the construction becomes correspondingly complicated.
It is an object of the present invention to provide an image forming apparatus in which even if an environmental fluctuation or the like occurs, the electrifying potential on an image bearing member is stabilized to thereby prevent the uneven density of an image.
It is another object of the present invention to provide an image forming apparatus comprising an image bearing member and a transferring member for transferring a toner image on the image bearing member onto a recording material in a transferring portion, wherein when the recording material is not present in the transferring portion, a voltage is applied to the transferring member, and on the basis of an electric current then flowing to the transferring member, a voltage applied to the transferring member when the non-image portion of the image bearing member is in the transferring portion is controlled.
Further objects of the present invention will become apparent from the following description.